n. (of an array)
a fixnum suitable for use as one of possibly several indices needed
to name an element of the array according to a multi-dimensional
Cartesian coordinate system. Such a fixnum must
be greater than or equal to zero,
and must be less than the corresponding dimension1
of the array.
(Unless otherwise explicitly specified,
the phrase "a list of valid array indices" further implies
that the length of the list must be the same as the
rank of the array.)
For a 2 by 3 array,
valid array indices for the first dimension are 0 and 1, and
valid array indices for the second dimension are 0, 1 and 2.